Stainless steel sheet for fuel cell separators, and production method therefor

ABSTRACT

A stainless steel sheet for fuel cell separators comprises: a predetermined chemical composition; and fine precipitates containing Cr and Ti at a steel sheet surface, wherein an average equivalent circular diameter of the fine precipitates is 20 nm or more and 500 nm or less, and a number of the fine precipitates existing per 1 μm2 at the steel sheet surface is three or more.

TECHNICAL FIELD

The present disclosure relates to a stainless steel sheet for fuel cellseparators that has excellent contact electric resistance (hereafteralso referred to as “contact resistance”) and corrosion resistance, anda production method therefor.

BACKGROUND

In recent years, fuel cells that have excellent generation efficiencyand emit no carbon dioxide are being developed for global environmentprotection. Such a fuel cell generates electricity from hydrogen andoxygen through an electrochemical reaction. The fuel cell has asandwich-like basic structure, and includes an electrolyte membrane(ion-exchange membrane), two electrodes (fuel electrode and airelectrode), gas diffusion layers of oxygen (air) and hydrogen, and twoseparators.

Fuel cells are classified as phosphoric acid fuel cells, moltencarbonate fuel cells, solid oxide fuel cells, alkaline fuel cells, andpolymer electrolyte fuel cells (PEFC: proton-exchange membrane fuelcells or polymer electrolyte fuel cells) according to the type ofelectrolyte membrane used, which are each being developed.

Of these fuel cells, polymer electrolyte fuel cells have, for example,the following advantages over other fuel cells.

(a) The fuel cell operating temperature is about 80° C., so thatelectricity can be generated at significantly low temperature.

(b) The fuel cell body can be reduced in weight and size.

(c) The fuel cell can be started promptly, and has high fuel efficiencyand power density.

Polymer electrolyte fuel cells are therefore expected to be used aspower sources in electric vehicles, home or industrial stationarygenerators, and portable small generators.

A polymer electrolyte fuel cell extracts electricity from hydrogen andoxygen via a polymer membrane. As illustrated in FIG. 1, amembrane-electrode joined body 1 is sandwiched between gas diffusionlayers 2 and 3 (for example, carbon paper) and separators (bipolarplates) 4 and 5, forming a single component (a single cell). Anelectromotive force is generated between the separators 4 and 5.

The membrane-electrode joined body 1 is called a membrane-electrodeassembly (MEA). The membrane-electrode joined body 1 is an assembly of apolymer membrane and an electrode material such as carbon black carryinga platinum catalyst on the front and back surfaces of the membrane, andhas a thickness of several 10 μm to several 100 μm. The gas diffusionlayers 2 and 3 are often integrated with the membrane-electrode joinedbody 1.

In the case of actually using polymer electrolyte fuel cells, severaltens to hundreds of single cells such as the above are typicallyconnected in series to form a fuel cell stack and put to use.

The separators 4 and 5 are required to function not only as

(a) partition walls separating single cells, but also as

(b) conductors carrying generated electrons,

(c) air passages 6 through which oxygen (air) flows and hydrogenpassages 7 through which hydrogen flows, and

(d) exhaust passages through which generated water or gas is exhausted(the air passages 6 or the hydrogen passages 7 also serve as the exhaustpassages).

The separators therefore need to have excellent durability and electricconductivity.

Regarding durability, about 5000 hours are expected in the case of usingthe polymer electrolyte fuel cell as a power source in an electricvehicle, and about 40000 hours are expected in the case of using thepolymer electrolyte fuel cell as a home stationary generator or thelike. Since the proton conductivity of the polymer membrane (electrolytemembrane) decreases if metal ions are eluted due to corrosion, theseparators need to be durable for long-term generation.

Regarding electric conductivity, the contact resistance between theseparator and the gas diffusion layer is desirably as low as possible,because an increase in contact resistance between the separator and thegas diffusion layer causes lower generation efficiency of the polymerelectrolyte fuel cell. A lower contact resistance between the separatorand the gas diffusion layer contributes to better power generationproperty.

Polymer electrolyte fuel cells using graphite as separators have alreadybeen in practical use. The separators made of graphite are advantageousin that the contact resistance is relatively low and also corrosion doesnot occur. The separators made of graphite, however, easily break onimpact, and so are disadvantageous in that the size reduction isdifficult and the processing cost for forming gas flow passages is high.These drawbacks of the separators made of graphite hinder the widespreaduse of polymer electrolyte fuel cells.

Attempts have been made to use a metal material as the separatormaterial instead of graphite. In particular, various studies have beenconducted to commercialize separators made of stainless steel, titanium,a titanium alloy, or the like for enhanced durability and lower contactresistance.

For example, JP H8-180883 A (PTL 1) discloses a technique of using, asseparators, a metal such as stainless steel or a titanium alloy thateasily forms a passive film. With the technique disclosed in PTL 1,however, the formation of the passive film causes an increase in contactresistance, and leads to lower generation efficiency. The metal materialdisclosed in PTL 1 thus has problems such as high contact resistance ascompared with the graphite material.

JP H10-228914 A (PTL 2) discloses a technique of plating the surface ofa metal separator such as an austenitic stainless steel sheet (SUS304)with gold to reduce the contact resistance and ensure high output.However, gold plating incurs higher cost.

JP 2000-328200 A (PTL 3) and JP 2007-12634 A (PTL 4) disclose techniquesof exposing a metal boride at the surface of stainless steel to reducethe contact resistance. These techniques, however, require the additionof a large amount of B, C, and the like as a steel component, so thatworkability decreases. Besides, since a large precipitate is exposed atthe steel surface, cracking, rough surface, and the like tend tooriginate from the coarse precipitate when working the steel into aseparator shape. Furthermore, the reduction in contact resistance isinsufficient.

CITATION LIST Patent Literatures

PTL 1: JP H8-180883 A

PTL 2: JP H10-228914 A

PTL 3: JP 2000-328200 A

PTL 4: JP 2007-12634 A

SUMMARY Technical Problem

It could therefore be helpful to provide a stainless steel sheet forfuel cell separators that has excellent contact resistance and corrosionresistance and also has sufficient workability at low cost.

It could also be helpful to provide a production method for thestainless steel sheet for fuel cell separators.

Solution to Problem

We conducted extensive examination to improve contact resistance whileensuring various properties of a stainless steel sheet for fuel cellseparators, in particular corrosion resistance and workability.

The stainless steel has a passive film at its surface. This passive filmcauses an increase in contact resistance when the stainless steel sheetis used as a fuel cell separator.

Hence, we first attempted to reduce the contact resistance in thefollowing manner: Various precipitates are formed in the steel surfacelayer, and such precipitates are exposed at the steel surface, to bringthe stainless steel sheet constituting a separator and a fuel cellcomponent member such as a gas diffusion layer into contact with eachother without the passive film therebetween.

We consequently discovered that an effective way of reducing the contactresistance while ensuring workability and corrosion resistance is toinclude Ti in the chemical composition of the steel sheet and use aprecipitate containing Ti and Cr.

However, even with use of such a precipitate (hereafter also referred toas “Cr and Ti precipitate”), the contact resistance cannot be reducedsatisfactorily in some cases.

Accordingly, we conducted further examination based on theabove-mentioned discoveries, and discovered the following:

-   -   By finely and densely dispersing the Cr and Ti precipitates at        the steel sheet surface, that is, by limiting the average        equivalent circular diameter of the Cr and Ti precipitates at        the steel sheet surface to 20 nm or more and 500 nm or less and        the number of the precipitates existing per 1 μm² at the surface        to three or more, the contact resistance can be further reduced        while ensuring corrosion resistance and workability.    -   To control the precipitation form of the precipitates as stated        above, it is important to control the chemical composition and        the production conditions. In particular, it is important to        include Ti in the chemical composition and optimize the        annealing atmosphere.    -   To cause the precipitates to exist at the steel sheet surface,        it is important to etch, by anodic electrolysis, the surface of        an annealed sheet obtained as a result of annealing. Especially,        by limiting the etching amount to a predetermined range through        the total electric charge applied, the precipitates can be        sufficiently exposed at the steel sheet surface, with it being        possible to reduce the contact resistance more advantageously.

The reason why finely and densely dispersing the Cr and Ti precipitatesat the steel sheet surface in the above-mentioned manner enables furtherreduction in contact resistance is considered as follows.

By finely and densely dispersing the Cr and Ti precipitates at the steelsheet surface, a current path not involving the passive film can beobtained uniformly and abundantly over the entire surface of thestainless steel sheet constituting a separator, as a result of which thecontact resistance can be reduced considerably.

When the stainless steel sheet is put in an especially severe corrosiveenvironment in practical use or subjected to heat treatment in a fuelcell stack production process, the passive film at the surface of thestainless steel sheet grows thick, and, in some cases, the passive filmgrows to such a thickness that can be regarded substantially as an oxidelayer, and the contact resistance increases. We conducted furtherexamination to maintain low contact resistance even in such cases.

We consequently discovered that, by increasing the atomic concentrationof Cr existing in chemical form other than metal in the passive film atthe steel sheet surface while keeping the fine precipitates exposed atthe steel sheet surface, that is, by setting the ratio of the atomicconcentration of Cr existing in chemical form other than metal to theatomic concentration of Fe existing in chemical form other than metal atthe steel sheet surface to 2.0 or more, low contact resistance can bemaintained more advantageously even in the case where the steel sheet isput in a severe corrosive environment or subjected to heat treatment ina fuel cell stack production process.

The reason for this is considered as follows. By increasing the atomicconcentration of Cr existing in chemical form other than metal in thepassive film at the steel sheet surface, the growth (thickening) of thepassive film at the steel sheet surface is inhibited even when the steelsheet is exposed to the above-mentioned heat treatment environment, andas a result the exposure state of the Cr and Ti precipitates at thesteel sheet surface is maintained favorably.

The present disclosure is based on these discoveries and furtherstudies.

We thus provide:

1. A stainless steel sheet for fuel cell separators, comprising: achemical composition containing (consisting of), in mass %, C: 0.003% to0.030%, Si: 0.01% to 1.00%, Mn: 0.01% to 1.00%, P: 0.050% or less, S:0.030% or less, Cr: 16.0% to 32.0%, Ni: 0.01% to 1.00%, Ti: 0.05% to0.45%, Al: 0.001% to 0.200%, and N: 0.030% or less, with the balancebeing Fe and inevitable impurities; and fine precipitates containing Crand Ti at a steel sheet surface, wherein an average equivalent circulardiameter of the fine precipitates is 20 nm or more and 500 nm or less,and a number of the fine precipitates existing per 1 μm² at the steelsheet surface is three or more.

2. The stainless steel sheet for fuel cell separators according to 1.,wherein the chemical composition further contains, in mass %, one ormore selected from Mo: 0.01% to 2.50%, Cu: 0.01% to 0.80%, Co: 0.01% to0.50%, and W: 0.01% to 3.00%.

3. The stainless steel sheet for fuel cell separators according to 1. or2., wherein the chemical composition further contains, in mass %, one ormore selected from Nb: 0.01% to 0.60%, Zr: 0.01% to 0.30%, V: 0.01% to0.30%, Ca: 0.0003% to 0.0030%, Mg: 0.0005% to 0.0050%, B: 0.0003% to0.0050%, REM (rare earth metal): 0.001% to 0.100%, Sn: 0.001% to 0.500%,and Sb: 0.001% to 0.500%.

4. The stainless steel sheet for fuel cell separators according to anyof 1. to 3., wherein a ratio [Cr]/[Fe] of an atomic concentration of Crexisting in chemical form other than metal to an atomic concentration ofFe existing in chemical form other than metal at the steel sheet surfaceis 2.0 or more.

5. A production method for a stainless steel sheet for fuel cellseparators, comprising: preparing a stainless steel sheet having thechemical composition according to any of 1. to 3., as a material;subjecting the stainless steel sheet to annealing, to obtain an annealedsheet; and subjecting the annealed sheet to anodic electrolysis, whereinan atmosphere in the annealing has a dew point of −35° C. or less and anitrogen concentration of 1 vol % or more, and a total electric chargeapplied in the anodic electrolysis is 5 C/dm² to 60 C/dm².

6. The production method for a stainless steel sheet for fuel cellseparators according to 5., further comprising after the anodicelectrolysis, subjecting the annealed sheet to Cr condensationtreatment, the Cr condensation treatment being immersion in an oxidizingsolution or electrolysis in a potential range in which the stainlesssteel sheet is passivated.

Advantageous Effect

It is possible to obtain a stainless steel sheet for fuel cellseparators that has excellent contact resistance while ensuringcorrosion resistance and workability at low cost.

In particular, favorable contact resistance property can be maintainedeven in the case where the steel sheet is put in an especially severecorrosive environment in practical use or subjected to heat treatment ina fuel cell stack production process.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram illustrating the basic structure of a fuelcell;

FIG. 2 is a diagram illustrating an example of a secondary electronimage obtained by observing a steel sheet surface after annealing by ascanning electron microscope in sample No. 2 in the examples;

FIG. 3 is a diagram illustrating an example of a secondary electronimage obtained by observing a steel sheet surface after anodicelectrolysis by a scanning electron microscope in sample No. 2 in theexamples; and

FIG. 4 is a diagram illustrating an example of an EDX spectrum of fineprecipitates formed at the steel sheet surface after anodic electrolysisin sample No. 2 in the examples.

DETAILED DESCRIPTION

A presently disclosed stainless steel sheet for fuel cell separators isdescribed in detail below.

(1) Chemical Composition

The reasons for limiting the chemical composition of the presentlydisclosed stainless steel sheet for fuel cell separators to the rangedescribed above are given below. While the unit of the content of eachelement in the chemical composition is “mass %,” the unit is hereaftersimply expressed by “%” unless otherwise specified.

C: 0.003% to 0.030%

Higher C content improves strength, and lower C content improvesworkability and corrosion resistance. To achieve sufficient strength,the C content needs to be 0.003% or more. If the C content is more than0.030%, workability and corrosion resistance decrease markedly. The Ccontent is therefore in a range of 0.003% to 0.030%. The C content ispreferably 0.005% or more. The C content is preferably 0.020% or less,more preferably 0.015% or less, and further preferably 0.010% or less.

Si: 0.01% to 1.00%

Si is an element useful as a deoxidizer. This effect is achieved with aSi content of 0.01% or more. If the Si content is more than 1.00%,workability decreases markedly, and it is difficult to work the steelsheet into a separator. The Si content is therefore in a range of 0.01%to 1.00%. The Si content is preferably 0.10% or more. The Si content ispreferably 0.50% or less, and more preferably 0.20% or less.

Mn: 0.01% to 1.00%

Mn has a deoxidation action. This effect is achieved with a Mn contentof 0.01% or more. If the Mn content is more than 1.00%, workability andcorrosion resistance decrease. The Mn content is therefore in a range of0.01% to 1.00%. The Mn content is preferably 0.10% or more. The Mncontent is preferably 0.25% or less, and more preferably 0.20% or less.

P: 0.050% or Less

P is an element that decreases corrosion resistance. Moreover, Psegregates to crystal grain boundaries and thus decreases hotworkability. Accordingly, the P content is desirably as low as possible,and is limited to 0.050% or less. The P content is preferably 0.040% orless. The P content is further preferably 0.030% or less. No lower limitis placed on the P content, yet the P content is preferably 0.005% ormore because excessive dephosphorization incurs higher cost.

S: 0.030% or less

S accelerates the precipitation of MnS, and decreases corrosionresistance. Accordingly, the S content is desirably low, and is limitedto 0.030% or less. The S content is preferably 0.010% or less. The Scontent is further preferably 0.004% or less. No lower limit is placedon the S content, yet the S content is preferably 0.001% or more becauseexcessive desulfurization incurs higher cost.

Cr: 16.0% to 32.0%

Cr is an important element to ensure the corrosion resistance of thestainless steel. Cr is also an important element that forms a nitride, acarbide, a carbonitride, an oxide, or a mixture thereof during annealingto exist at the surface as a precipitate together with Ti, thusimproving electric conductivity and reducing contact resistance. If theCr content is less than 16.0%, corrosion resistance required of fuelcell separators cannot be obtained. If the Cr content is 16.0% or morewhile the Ti content is appropriately controlled as described below, asufficient amount of fine precipitates containing Cr and Ti can beformed at the steel sheet surface, and as a result electric conductivityrequired of fuel cell separators can be obtained. If the Cr content ismore than 32.0%, workability decreases. The Cr content is therefore in arange of 16.0% to 32.0%. The Cr content is preferably 18.0% or more, andmore preferably 20.0% or more. The Cr content is preferably 26.0% orless, and more preferably 24.0% or less.

Ni: 0.01% to 1.00%

Ni is an element that effectively contributes to improved toughness andcrevice corrosion resistance. This effect is achieved with a Ni contentof 0.01% or more. If the Ni content is more than 1.00%, stress corrosioncrack sensitivity increases. Besides, higher cost is incurred because Niis an expensive element. The Ni content is therefore in a range of 0.01%to 1.00%. The Ni content is preferably 0.10% or more. The Ni content ispreferably 0.50% or less, and more preferably 0.30% or less.

Ti: 0.05% to 0.45%

Ti is an important element that forms a nitride, a carbide, acarbonitride, an oxide, or a mixture thereof during annealing to existat the surface as a precipitate together with Cr, thus improvingelectric conductivity and reducing contact resistance. In particular,since Ti nitride has high electric conductivity, contact resistance canbe reduced effectively by causing such Ti nitride to exist at the steelsurface as a fine precipitate containing Cr and Ti. This effect isachieved with a Ti content of 0.05% or more. If the Ti content is morethan 0.45%, workability decreases. The Ti content is therefore in arange of 0.05% to 0.45%. The Ti content is preferably 0.10% or more,more preferably 0.15% or more, and further preferably 0.20% or more. TheTi content is preferably 0.40% or less, more preferably 0.35% or less,and further preferably 0.30% or less.

Al: 0.001% to 0.200%

Al is an element useful for deoxidation. This effect is achieved with aAl content of 0.001% or more. If the Al content is more than 0.200%, Alundergoes oxidation or nitriding preferentially during annealing, and alayer mainly composed of Al tends to form at the steel surface. Thissuppresses the formation of fine precipitates containing Cr and Ti. TheAl content is therefore in a range of 0.001% to 0.200%. The Al contentis preferably 0.010% or more, more preferably 0.020% or more, andfurther preferably 0.030% or more. The Al content is preferably 0.150%or less, more preferably 0.100% or less, and further preferably 0.050%or less.

N: 0.030% or Less

If the N content is more than 0.030%, corrosion resistance andworkability decrease markedly. The N content is therefore 0.030% orless. The N content is preferably 0.020% or less. The N content is morepreferably 0.015% or less. No lower limit is placed on the N content,yet the N content is preferably 0.003% or more because excessivedenitriding incurs higher cost.

While the basic components have been described above, the presentlydisclosed stainless steel sheet for fuel cell separators may optionallycontain the following elements as appropriate.

Mo: 0.01% to 2.50%

Mo stabilizes the passive film of the stainless steel and improvescorrosion resistance. This effect is achieved with a Mo content of 0.01%or more. If the Mo content is more than 2.50%, workability decreases.Accordingly, in the case of containing Mo, the Mo content is in a rangeof 0.01% to 2.50%. The Mo content is preferably 0.50% or more, and morepreferably 1.00% or more. The Mo content is preferably 2.00% or less.

Cu: 0.01% to 0.80%

Cu is an element that enhances corrosion resistance. This effect isachieved with a Cu content of 0.01% or more. If the Cu content is morethan 0.80%, hot workability decreases. Accordingly, in the case ofcontaining Cu, the Cu content is in a range of 0.01% to 0.80%. The Cucontent is preferably 0.10% or more. The Cu content is preferably 0.60%or less. The Cu content is more preferably 0.45% or less.

Co: 0.01% to 0.50%

Co is an element that enhances corrosion resistance. This effect isachieved with a Co content of 0.01% or more. If the Co content is morethan 0.50%, workability decreases. Accordingly, in the case ofcontaining Co, the Co content is in a range of 0.01% to 0.50%. The Cocontent is preferably 0.10% or more. The Co content is preferably 0.30%or less.

W: 0.01% to 3.00%

W is an element that enhances corrosion resistance. This effect isachieved with a W content of 0.01% or more. If the W content is morethan 3.00%, workability decreases. Accordingly, in the case ofcontaining W, the W content is in a range of 0.01% to 3.00%. The Wcontent is preferably 0.10% or more. The W content is preferably 0.80%or less, and more preferably 0.60% or less.

Nb: 0.01% to 0.60%

Nb is an element that combines with C and N to prevent excessiveprecipitation of Cr carbonitride in the steel and suppress a decrease incorrosion resistance (sensitization). These effects are achieved with aNb content of 0.01% or more. If the Nb content is more than 0.60%,workability decreases. Accordingly, in the case of containing Nb, the Nbcontent is in a range of 0.01% to 0.60%. The Nb content is preferably0.40% or less, and more preferably less than 0.20%.

Zr: 0.01% to 0.30%

Zr is an element that combines with C and N contained in the steel tosuppress sensitization, as with Nb. This effect is achieved with a Zrcontent of 0.01% or more. If the Zr content is more than 0.30%,workability decreases. Accordingly, in the case of containing Zr, the Zrcontent is in a range of 0.01% to 0.30%. The Zr content is preferably0.20% or less, more preferably 0.15% or less, and further preferably0.10% or less.

V: 0.01% to 0.30%

V is an element that combines with C and N contained in the steel andsuppresses a decrease in corrosion resistance (sensitization), as withNb and Zr. This effect is achieved with a V content of 0.01% or more. Ifthe V content is more than 0.30%, workability decreases. Accordingly, inthe case of containing V, the V content is in a range of 0.01% to 0.30%.The V content is preferably 0.20% or less, more preferably 0.15% orless, and further preferably 0.10% or less.

Ca: 0.0003% to 0.0030%

Ca improves castability and enhances manufacturability. This effect isachieved with a Ca content of 0.0003% or more. If the Ca content is morethan 0.0030%, Ca combines with S to form CaS, which causes a decrease incorrosion resistance. Accordingly, in the case of containing Ca, the Cacontent is in a range of 0.0003% to 0.0030%. The Ca content ispreferably 0.0005% or more. The Ca content is preferably 0.0020% orless.

Mg: 0.0005% to 0.0050%

Mg acts as a deoxidizer. This effect is achieved with a Mg content of0.0005% or more. If the Mg content is more than 0.0050%, the toughnessof the steel decreases, which can lead to a decrease inmanufacturability. Accordingly, in the case of containing Mg, the Mgcontent is in a range of 0.0005% to 0.0050%. The Mg content ispreferably 0.0020% or less.

B: 0.0003% to 0.0050%

B is an element that improves secondary working brittleness. This effectis achieved with a B content of 0.0003% or more. If the B content ismore than 0.0050%, a B-containing precipitate forms and workabilitydecreases. Accordingly, in the case of containing B, the B content is ina range of 0.0003% to 0.0050%. The B content is preferably 0.0005% ormore. The B content is preferably 0.0030% or less.

REM (Rare Earth Metal): 0.001% to 0.100%

REM (rare earth metal: elements of atomic numbers 57 to 71 such as La,Ce, and Nd) is an element effective for deoxidation. This effect isachieved with a REM content of 0.001% or more. If the REM content ismore than 0.100%, hot workability decreases. Accordingly, in the case ofcontaining REM, the REM content is in a range of 0.001% to 0.100%. TheREM content is preferably 0.010% or more. The REM content is preferably0.050% or less.

Sn: 0.001% to 0.500%

Sn is an element effective in preventing occurrence of rough surfacecaused by working. This effect is achieved with a Sn content of 0.001%or more. If the Sn content is more than 0.500%, hot workabilitydecreases. Accordingly, in the case of containing Sn, the Sn content isin a range of 0.001% to 0.500%. The Sn content is preferably 0.010% ormore. The Sn content is preferably 0.200% or less.

Sb: 0.001% to 0.500%

Sb is an element effective in preventing occurrence of rough surfacecaused by working, as with Sn. This effect is achieved with a Sb contentof 0.001% or more. If the Sb content is more than 0.500%, workabilitydecreases. Accordingly, in the case of containing Sb, the Sb content isin a range of 0.001% to 0.500%. The Sb content is preferably 0.010% ormore. The Sb content is preferably 0.200% or less.

The components other than those described above are Fe and inevitableimpurities.

As described above, the presently disclosed stainless steel sheet forfuel cell separators preferably has a chemical composition that, in mass%,

contains C: 0.003% to 0.030%, Si: 0.01% to 1.00%, Mn: 0.01% to 1.00%, P:0.050% or less, S: 0.030% or less, Cr: 16.0% to 32.0%, Ni: 0.01% to1.00%, Ti: 0.05% to 0.45%, Al: 0.001% to 0.200%, and N: 0.030% or less,

optionally contains one or more selected from Mo: 0.01% to 2.50%, Cu:0.01% to 0.80%, Co: 0.01% to 0.50%, and W: 0.01% to 3.00%, and

optionally contains one or more selected from Nb: 0.01% to 0.60%, Zr:0.01% to 0.30%, V: 0.01% to 0.30%, Ca: 0.0003% to 0.0030%, Mg: 0.0005%to 0.0050%, B: 0.0003% to 0.0050%, REM (rare earth metal): 0.001% to0.100%, Sn: 0.001% to 0.500%, and Sb: 0.001% to 0.500%,

with the balance being Fe and inevitable impurities.

(2) Fine Precipitate

It is very important that the presently disclosed stainless steel sheetfor fuel cell separators has fine precipitates containing Cr and Ti atits steel sheet surface, the average equivalent circular diameter of thefine precipitates is 20 nm or more and 500 nm or less, and the number ofthe fine precipitates existing per 1 μm² at the steel sheet surface isthree or more.

Fine precipitate at steel sheet surface: fine precipitate containing Crand Ti

The fine precipitate at the steel sheet surface is a fine precipitatecontaining Cr and Ti. By sufficiently exposing the fine precipitatescontaining Cr and Ti at the steel sheet surface, the contact resistancecan be reduced more advantageously.

Examples of the fine precipitate containing Cr and Ti include Cr and Tinitride, carbide, carbonitride, and oxide, and mixtures thereof.

The components of the fine precipitate can be determined from an EDXspectrum obtained by peeling the fine precipitate from the steel sheetsurface and analyzing the peeled fine precipitate using anenergy-dispersive X-ray spectrometer (EDX) attached to a transmissionelectron microscope (TEM).

Average Equivalent Circular Diameter of Fine Precipitates: 20 nm or Moreand 500 nm or Less

It is essential that the Cr and Ti precipitates are finely and denselydispersed at the steel sheet surface of the presently disclosedstainless steel sheet for fuel cell separators in order to reduce thecontact resistance, as mentioned above. In detail, it is important thatthe average equivalent circular diameter of the fine precipitates is 20nm or more and 500 nm or less.

If the average equivalent circular diameter is less than 20 nm, theprecipitates are refined excessively, so that the precipitates are notsufficiently exposed at the steel sheet surface from the passive film.In such a case, sufficient contact between the precipitate and a fuelcell component member such as a gas diffusion layer cannot be achieved,and desired contact resistance cannot be obtained. If the averageequivalent circular diameter is more than 500 nm, the precipitatescannot be finely and densely dispersed at the steel sheet surface, anddesired contact resistance cannot be obtained. Besides, cracking, roughsurface, and the like tend to originate from the precipitate whenworking the steel sheet into a desired separator shape.

Accordingly, the average equivalent circular diameter of the fineprecipitates is 20 nm or more and 500 nm or less. The average equivalentcircular diameter is preferably 30 nm or more, and more preferably 50 nmor more. The average equivalent circular diameter is preferably 200 nmor less, and more preferably 150 nm or less.

Number of Fine Precipitates Per 1 μm² at Steel Sheet Surface: Three orMore

It is also important that the number of the fine precipitates existingper 1 μm² at the steel sheet surface of the presently disclosedstainless steel sheet for fuel cell separators is three or more.

If the number of the fine precipitates per 1 μm² at the steel sheetsurface is less than three, the electrical contact point between thestainless steel sheet for separators and a fuel cell component part suchas a gas diffusion layer is insufficient, and desired contact resistancecannot be obtained. The number of the fine precipitates per 1 μm² at thesteel sheet surface is therefore three or more. The number is preferablyfive or more. The number is more preferably ten or more.

The average equivalent circular diameter of the fine precipitates andthe number of the fine precipitates per 1 μm² at the steel sheet surfacecan be determined as follows.

The steel sheet surface is observed for 10 observation fields with anaccelerating voltage of 3 kV and a magnification of 30000 times, using ascanning electron microscope (FE-SEM) equipped with a cold-cathode fieldemission electron gun. The equivalent circular diameter of eachprecipitate observed in the resultant secondary electron imagephotograph (SEM photograph) is measured, and their average is calculatedto find the average equivalent circular diameter of the fineprecipitates. A lower limit of 10 nm is placed on the particle size(equivalent circular diameter) of the precipitates measured here.

In addition, the number of the precipitates whose particle sizes havebeen measured as mentioned above is counted and the number of theprecipitates per 1 μm² is calculated for each observation field, andtheir average is calculated to find the number of the fine precipitatesper 1 μm² at the steel sheet surface.

Ratio of Atomic Concentration of Cr Existing in Chemical Form Other thanMetal to Atomic Concentration of Fe Existing in Chemical Form Other thanMetal at Steel Sheet Surface: 2.0 or More

By setting the ratio (hereafter also referred to as “[Cr]/[Fe]”) of theatomic concentration of Cr existing in chemical form other than metal tothe atomic concentration of Fe existing in chemical form other thanmetal at the surface of the stainless steel sheet to 2.0 or more, thegrowth of the passive film at the steel sheet surface is inhibited evenin the case where the steel sheet is put in a severe corrosiveenvironment or subjected to heat treatment in a fuel cell stackproduction process. Consequently, the exposure state of the Crprecipitates at the steel sheet surface is maintained, with it beingpossible to maintain low contact resistance. [Cr]/[Fe] is preferably 2.5or more.

Higher [Cr]/[Fe] is more advantageous in terms of inhibiting the growthof the passive film at the steel sheet surface, and so no upper limit isplaced on [Cr]/[Fe].

The “chemical form other than metal” denotes oxide and hydroxide. Indetail, for Cr, examples include CrO₂, Cr₂O₃, CrOOH, Cr(OH)₃, and CrO₃.For Fe, examples include FeO, Fe₃O₄, Fe₂O₃, and FeOOH.

[Cr]/[Fe] can be determined as follows.

The surface of the stainless steel sheet is measured by X-rayphotoelectron spectroscopy (hereafter also referred to as “XPS”), andthe obtained peaks of Cr and Fe are separated into the peaks of Cr andFe existing in metal chemical form and the peaks of Cr and Fe existingin chemical form other than metal. Dividing the atomic concentration ofCr existing in chemical form other than metal by the atomicconcentration of Fe existing in chemical form other than metalcalculated from the separated peaks yields [Cr]/[Fe].

In detail, a sample of 10 mm square was cut out of the steel sheet, andmeasured by an X-ray photoelectron spectrometer (AXIS-HS produced byShimadzu/Kratos Co.) with an extraction angle of 45 degrees using aAl-Kα monochromatic X-ray source. The peaks of Cr and Fe are separatedinto the peaks of Cr and Fe existing in metal chemical form and thepeaks of Cr and Fe existing in chemical form other than metal. Dividingthe atomic concentration of Cr existing in chemical form other thanmetal by the atomic concentration of Fe existing in chemical form otherthan metal calculated from the separated peaks yields [Cr]/[Fe].

Peak separation is performed by removing the background of the spectrumby Shirley method and using a Gauss-Lorentz complex function (proportionof Lorentz function: 30%).

In this measurement, Cr atoms existing as precipitates at the steelsheet surface might be measured simultaneously. The inclusion of such Cratoms existing as precipitates, however, poses no problem for thecalculation of [Cr]/[Fe].

In terms of the fuel cell stack installation space and content weight,the sheet thickness of the stainless steel sheet for fuel cellseparators is preferably in a range of 0.03 mm to 0.30 mm. If the sheetthickness is less than 0.03 mm, the production efficiency of thestainless steel sheet decreases. If the sheet thickness is more than0.30 mm, the stack installation space and weight increases. The sheetthickness is more preferably 0.10 mm or less.

(3) Production Method

A presently disclosed production method for a stainless steel sheet forfuel cell separators is described below.

The presently disclosed production method for a stainless steel sheetfor fuel cell separators includes: preparing a stainless steel sheethaving the chemical composition described above as a material;subjecting the stainless steel sheet to annealing, to obtain an annealedsheet; and subjecting the annealed sheet to anodic electrolysis.

Each process is described below.

Preparation

The preparation involves preparing a stainless steel sheet as amaterial. The stainless steel sheet as a material is not limited as longas it has the chemical composition described above.

For example, a stainless steel sheet having the chemical compositiondescribed above can be prepared by hot rolling a steel slab having thechemical composition described above to obtain a hot rolled sheet,optionally hot band annealing the hot rolled sheet, thereafter coldrolling the hot rolled sheet to obtain a cold rolled sheet with adesired sheet thickness, and further optionally subjecting the coldrolled sheet to intermediate annealing.

The conditions of hot rolling, cold rolling, hot band annealing,intermediate annealing, and the like are not limited, and may complywith conventional methods.

Annealing

The annealing involves annealing the stainless steel sheet as a materialprepared in the preparation to obtain an annealed sheet. It is importantto use an atmosphere of a dew point of −40° C. or less and a nitrogenconcentration of 1 vol % or more, in order to form desired fineprecipitates near the steel sheet surface.

Dew Point: −35° C. or Less

The dew point in the annealing needs to be −35° C. or less. A higher dewpoint facilitates an oxidation reaction. In particular, if the dew pointis more than −35° C., the oxide layer at the surface of the stainlesssteel sheet grows thick. This hinders the formation of the fineprecipitates containing Cr and Ti, and makes it impossible to obtaindesired contact resistance. Therefore, the dew point in the annealingneeds to be −35° C. or less. The dew point is preferably −40° C. orless, and more preferably −45° C. or less.

Nitrogen Concentration: 1 Vol % or More

To form the fine precipitates containing Cr and Ti described above, thenitrogen concentration of the atmosphere gas needs to be 1 vol % ormore. If the nitrogen concentration is less than 1 vol %, a necessaryamount of fine precipitates containing Cr and Ti cannot be formed, anddesired contact resistance cannot be obtained. The nitrogenconcentration is preferably 5 vol % or more, and more preferably 20 vol% or more.

Examples of atmosphere gas that can be used besides nitrogen includehydrogen gas, argon gas, helium gas, carbon monoxide gas, carbon dioxidegas, and ammonia gas.

Mixed gas of nitrogen gas and hydrogen gas is suitable, and ammoniadecomposition gas (hydrogen gas 75 vol %+nitrogen gas 25 vol %) isparticularly suitable.

By increasing the annealing temperature, the number of the fineprecipitates containing Cr and Ti can be increased. Moreover, theworkability can be improved to ease working into a separator shape.However, if the annealing temperature is excessively high, theequivalent circular diameter of the fine precipitates coarsens, anddesired contact resistance may be unable to be obtained. The annealingtemperature is therefore preferably 800° C. to 1100° C. The annealingtemperature is more preferably 850° C. or more. The annealingtemperature is more preferably 1050° C. or less.

The annealing conditions other than the above may comply withconventional methods.

Anodic Electrolysis

Total Electric Charge Applied: 5 C/dm² to 60 C/dm²

The anodic electrolysis involves subjecting the annealed sheet obtainedin the annealing to anodic electrolysis. In the anodic electrolysis, itis important to appropriately control the etching amount so that thefine precipitates near the surface of the steel sheet formed by theannealing are exposed at the steel sheet surface without dropping off.The etching amount (the amount of the stainless steel sheet dissolved)is controlled by the total electric charge applied.

If the total electric charge applied is less than 5 C/dm², the fineprecipitates are not sufficiently exposed at the steel sheet surface,which makes it difficult to obtain the desired contact resistance. Ifthe total electric charge applied is more than 60 C/dm², the etchingamount is excessively high, and the fine precipitates formed near thesurface layer drop off, which makes it difficult to obtain the desiredcontact resistance. Therefore, the total electric charge applied in theanodic electrolysis is in a range of 5 C/dm² to 60 C/dm². The totalelectric charge applied is preferably 10 C/dm² or more, and morepreferably 15 C/dm² or more. The total electric charge applied ispreferably 40 C/dm² or less, and more preferably 25 C/dm² or less.

As an electrolytic solution, a sulfuric acid aqueous solution, a nitricacid aqueous solution, a phosphoric acid aqueous solution, a sodiumsulfate aqueous solution, or the like is suitably used. The anodicelectrolysis conditions other than the above are not limited as long asthe total electric charge applied can be adjusted as described above,and may comply with conventional methods.

Condensation Treatment for Cr Existing in Chemical Form Other than Metalat Steel Sheet Surface

After the anodic electrolysis, treatment (hereafter also referred to as“Cr condensation treatment”) of condensing Cr existing in chemical formother than metal at the steel sheet surface, i.e. Cr existing inchemical form other than metal in the passive film, may be furtherperformed. The Cr condensation treatment can increase the ratio([Cr]/[Fe]) of the atomic concentration of Cr existing in chemical formother than metal to the atomic concentration of Fe existing in chemicalform other than metal at the steel sheet surface.

Examples of the Cr condensation treatment include immersion in anoxidizing solution and electrolysis in a potential range in which thestainless steel sheet is passivated.

Examples of the oxidizing solution include a nitric acid aqueoussolution and a hydrogen peroxide aqueous solution. A longer immersiontime facilitates the condensation of Cr in the passive film. However, ifthe immersion time is excessively long, the effect is saturated andproductivity decreases. Accordingly, the immersion time is preferably 1min or more and 2 hr (120 min) or less.

In the case of using a nitric acid aqueous solution, the concentrationof nitric acid is preferably 10 g/L to 400 g/L. The treatmenttemperature is not limited, but is preferably 30° C. to 60° C.

In the electrolysis, the potential may be adjusted to such a potentialrange in which the stainless steel sheet is passivated. In particular,it is preferable to adjust the potential to such a potential range inwhich components such as Fe and Ni other than Cr in the steel aredissolved and Cr is not dissolved.

The potential range (passivation area) in which the stainless steelsheet is passivated varies depending on the electrolytic solution usedand the chemical composition of the stainless steel sheet. It istherefore preferable to adjust the potential in each case. For example,in the case of using a 50 g/L nitric acid aqueous solution, electrolysisis preferably performed in a potential range of 0.4 V to 0.8 V (vs.Ag/AgCl). A longer electrolysis time facilitates the condensation of Crexisting in chemical form other than metal in the passive film. However,if the electrolysis time is excessively long, the effect is saturatedand productivity decreases. Accordingly, the electrolysis time ispreferably 1 min or more and 2 hr (120 min) or less.

Other Treatments

Treatment of roughening the steel sheet surface may be performed beforethe annealing. By making the steel sheet surface rough beforehand, thecontact resistance reduction effect can be further enhanced. Forexample, hydrofluoric acid aqueous solution immersion, shot blasting, ormechanical polishing is suitable.

EXAMPLES Example 1

Cold rolled sheets of stainless steels of 0.08 mm in sheet thicknesshaving the respective compositions listed in Table 1 were prepared, andsubjected to annealing under the conditions listed in Table 2. In Table2, the annealing temperature is the temperature measured at the steelsheet surface, and the annealing time is the residence time in atemperature range of “annealing temperature−10° C.” to “annealingtemperature”.

After this, anodic electrolysis was performed in a 30 g/L sulfuric acidaqueous solution at a temperature of 40° C. so as to have the totalelectric charge applied listed in Table 2, thus obtaining a stainlesssteel sheet for separators. Here, sample No. 15 was not subjected toanodic electrolysis.

The contact resistance and corrosion resistance of each resultantstainless steel sheet for separators were evaluated as follows.

(1) Evaluation of Contact Resistance

Regarding the contact resistance, a sample was sandwiched between sheetsof carbon paper (TGP-H-120 produced by Toray Industries, Inc.), andfurther contacted from both sides by Au plated Cu electrodes. A pressureof 0.98 MPa (=10 kg/cm²) per unit area was applied to cause current toflow, and the voltage difference between the electrodes was measured tocalculate the electric resistance. The value obtained by multiplying themeasured electric resistance by the area of the contact surface wastaken to be the contact resistance value, and the contact resistance wasevaluated based on the following criteria. The results are shown inTable 2.

Good: 30 mΩ·cm² or less

Poor: more than 30 mΩ·cm².

(2) Evaluation of Corrosion Resistance

Typically, stainless steel is more susceptible to transpassivedissolution and suffers greater degradation in corrosion resistance whenthe applied potential is higher. To evaluate the stability in the eventof long exposure to high potential in a separator use environment, eachsample was immersed in a sulfuric acid aqueous solution of a temperatureof 80° C. and a pH of 3 and subjected to the application of a constantpotential of 0.9 V (vs. SHE) for 20 hours using Ag/AgCl (saturated KClaqueous solution) as a reference electrode, and the current densityafter 20 hours was measured. Based on the current density after 20hours, the corrosion resistance was evaluated based on the followingcriteria. The results are shown in Table 2.

Good: 1 μA/cm² or less

Poor: more than 1 μA/cm².

The average equivalent circular diameter of the fine precipitates andthe number of the fine precipitates per 1 μm² at the steel sheet surfacewere measured by the above-mentioned methods. S-4100 produced byHitachi, Ltd. was used as a scanning electron microscope (FE-SEM)equipped with a cold-cathode field emission electron gun. The resultsare shown in Table 2.

For reference, FIG. 2 illustrates an example of a secondary electronimage obtained by observing, by a scanning electron microscope equippedwith a cold-cathode field emission electron gun, the steel sheet surfaceafter annealing with an accelerating voltage of 3 kV and a magnificationof 30000 times in sample No. 2, and FIG. 3 illustrates an example of asecondary electron image obtained by observing the steel sheet surfaceafter anodic electrolysis in the same sample No. 2. As illustrated inFIGS. 2 and 3, the outlines (white color regions) of the fineprecipitates were not clear and the fine precipitates were notsufficiently exposed from the passive film surface after the annealing(before the anodic electrolysis), but the fine precipitates were exposedafter the anodic electrolysis.

Moreover, the exposed fine precipitates were peeled from the samplesurface. The peeled fine precipitates were fixed to a Cu mesh by carbonvapor deposition, and analyzed using an energy-dispersive X-rayspectrometer (EDX) attached to a transmission electron microscope (TEM,JEM2010 produced by JEOL Ltd.). From the resultant EDX spectrum, thecomponents of the exposed fine precipitates were determined. The resultsare shown in Table 2.

For reference, FIG. 4 illustrates an example of an EDX spectrum of thefine precipitates formed at the steel sheet surface after anodicelectrolysis in sample No. 2. As illustrated in FIG. 4, the fineprecipitates at the steel sheet surface contained Cr and Ti.

TABLE 1 Steel sample Chemical composition (mass %) ID C Si Mn P S Cr NiTi Al N Mo Cu Nb Other components Remarks A 0.007 0.18 0.15 0.028 0.00220.9 0.19 0.31 0.032 0.011 — 0.44 — — Conforming steel B 0.006 0.08 0.110.033 0.001 20.9 0.15 0.26 0.039 0.009 — — — — Conforming steel C 0.0120.12 0.17 0.021 0.003 21.0 0.13 0.33 0.033 0.011 0.53 — — Co: 0.02,Conforming steel V: 0.04, Ca: 0.0004, B: 0.0005 D 0.007 0.09 0.15 0.0240.002 23.9 0.13 0.35 0.107 0.016 1.07 — 0.09 — Conforming steel E 0.0040.20 0.16 0.027 0.009 25.8 0.27 0.10 0.072 0.013 — — 0.14 — Conformingsteel F 0.005 0.11 0.13 0.024 0.001 23.4 0.29 0.32 0.092 0.011 1.01 0.060.11 W: 0.02, Conforming steel Zr: 0.03, Mg: 0.0009, REM: 0.003, Sn:0.012, Sb: 0.025 G 0.005 0.18 0.16 0.026 0.007 30.3 0.22 0.01 0.0790.012 1.80 — 0.14 — Comparative steel H 0.007 0.13 0.12 0.029 0.002 20.60.24 0.28 0.174 0.010 — — — — Conforming steel I 0.004 0.08 0.18 0.0180.001 28.5 0.18 0.21 0.014 0.006 — — — — Conforming steel J 0.008 0.070.10 0.029 0.003 17.8 0.32 0.26 0.039 0.009 1.12 — — — Conforming steelK 0.009 0.14 0.18 0.027 0.002 20.8 0.21 0.29 0.033 0.009 0.06 0.43 — V:0.03, Ca: 0.0006, Conforming steel B: 0.0004 L 0.008 0.11 0.19 0.0310.001 20.6 0.14 0.28 0.028 0.008 — — — Co: 0.04, V: 0.05, Conformingsteel Ca: 0.0008, B: 0.0008 M 0.003 0.10 0.08 0.026 0.001 28.4 0.28 0.030.098 0.005 — — — — Comparative steel N 0.021 0.58 0.55 0.025 0.001 20.80.52 0.32 0.031 0.011 — — — — Conforming steel

TABLE 2 Fine precipitates Sample production conditions at steel sheetsurface Anodic Average Annealing electrolysis equivalent Number of SteelDew Annealing Annealing Total electric circular fine Sample sample pointtemperature time charge applied Precipitate diameter precipitates No. IDAtmosphere gas composition (° C.) (° C.) (sec) (C/dm²) components (nm)per 1 μm² 1 A hydrogen 75 vol % + nitrogen 25 vol % −52 950 5 10 Cr, Ti80 13 2 hydrogen 75 vol % + nitrogen 25 vol % −52 950 5 20 Cr, Ti 85 273 hydrogen 75 vol % + nitrogen 25 vol % −52 950 5 30 Cr, Ti 95 24 4hydrogen 75 vol % + nitrogen 25 vol % −52 950 5 40 Cr, Ti 90 11 5 Bhydrogen 75 vol % + nitrogen 25 vol % −60 950 5 20 Cr, Ti 90 24 6 Chydrogen 90 vol % + nitrogen 10 vol % −62 970 10 20 Cr, Ti 85 22 7 Dhydrogen 75 vol % + nitrogen 25 vol % −58 980 10 20 Cr, Ti 100 28 8 Ehydrogen 75 vol % + nitrogen 25 vol % −55 980 30 20 Cr, Ti 150 5 9 Fhydrogen 75 vol % + nitrogen 25 vol % −55 980 10 20 Cr, Ti 95 25 10 Ghydrogen 75 vol % + nitrogen 25 vol % −45 980 10 20 Cr 40 2 11 Nhydrogen 75 vol % + nitrogen 25 vol % −48 950 5 30 Cr, Ti 90 25 12 Ahydrogen 100 vol % −60 950 5 20 — — 0 13 hydrogen 75 vol % + nitrogen 25vol % −52 950 5 2.5 Cr, Ti 60 1 14 hydrogen 75 vol % + nitrogen 25 vol %−52 950 5 80 — — 0 15 hydrogen 75 vol % + nitrogen 25 vol % −52 950 5 NoCr, Ti 70 1 16 H hydrogen 95 vol % + nitrogen 5 vol % −53 930 5 20 Cr,Ti 60 21 17 I hydrogen 75 vol % + nitrogen 25 vol % −61 980 5 30 Cr, Ti75 23 18 J hydrogen 75 vol % + nitrogen 25 vol % −38 980 5 15 Cr, Ti 8021 19 K hydrogen 75 vol % + nitrogen 25 vol % −51 950 5 20 Cr, Ti 80 2520 L hydrogen 95 vol % + nitrogen 5 vol % −46 950 5 20 Cr, Ti 65 20 21 Mhydrogen 75 vol % + nitrogen 25 vol % −51 950 5 20 Cr 30 1 22 B hydrogen75 vol % + nitrogen 25 vol % −33 920 5 10 Cr, Ti 70 2 Evaluation resultContact resistance Corrosion resistance Sample No. (mΩ · cm²)Determination (μA/cm²) Determination Remarks  1 19.7 Good 0.17 GoodExample  2 14.5 Good 0.17 Good Example  3 15.7 Good 0.17 Good Example  416.1 Good 0.17 Good Example  5 16.8 Good 0.19 Good Example  6 17.3 Good0.16 Good Example  7 14.4 Good 0.15 Good Example  8 28.6 Good 0.13 GoodExample  9 15.2 Good 0.15 Good Example 10 35.4 Poor 0.13 GoodComparative Example 11 15.6 Good 0.17 Good Example 12 36.7 Poor 0.17Good Comparative Example 13 141.8 Poor 0.17 Good Comparative Example 1436.7 Poor 0.17 Good Comparative Example 15 485.0 Poor 0.17 GoodComparative Example 16 18.4 Good 0.17 Good Example 17 16.4 Good 0.13Good Example 18 17.5 Good 0.25 Good Example 19 16.3 Good 0.17 GoodExample 20 18.5 Good 0.17 Good Example 21 36.8 Poor 0.13 GoodComparative Example 22 31.1 Poor 0.17 Good Comparative Example

The table reveals the following points.

(a) All Examples had desired contact resistance and corrosionresistance.

(b) In the samples of Comparative Examples No. 10 and 21, the Ti contentwas low. Consequently, the fine precipitates containing Cr and Ti werenot sufficiently formed at the steel sheet surface, and desired contactresistance was not obtained.

(c) In the sample of Comparative Example No. 12, the atmosphere in theannealing did not contain nitrogen. Consequently, the fine precipitatescontaining Cr and Ti were not sufficiently formed, and desired contactresistance was not obtained.

(d) In the sample of Comparative Example No. 13, the total electriccharge applied in the anodic electrolysis was insufficient.Consequently, the fine precipitates containing Cr and Ti were notsufficiently exposed at the steel sheet surface, and the number of thefine precipitates at the steel sheet surface was insufficient, so thatdesired contact resistance was not obtained.

(e) In the sample of Comparative Example No. 14, the total electriccharge applied in the anodic electrolysis was excessively high.Consequently, the fine precipitates containing Cr and Ti dropped off thesteel sheet surface, and desired contact resistance was not obtained.

(f) In the sample of Comparative Example No. 15, no anodic electrolysiswas performed. Consequently, the fine precipitates containing Cr and Tiwere not exposed at the steel sheet surface, and the number of the fineprecipitates at the steel sheet surface was insufficient, so thatdesired contact resistance was not obtained.

(g) In the sample of Comparative Example No. 22, the dew point in theannealing was high. Consequently, the fine precipitates containing Crand Ti were not sufficiently formed, and desired contact resistance wasnot obtained.

Example 2

Cold rolled sheets of stainless steels of 0.08 mm in sheet thicknesshaving the respective compositions listed in Table 1 were prepared, andsubjected to annealing under the conditions listed in Table 3. In Table3, the annealing temperature is the temperature measured at the steelsheet surface, and the annealing time is the residence time in atemperature range of “annealing temperature−10° C.” to “annealingtemperature”.

After this, anodic electrolysis was performed in a 30 g/L sulfuric acidaqueous solution at a temperature of 40° C. so as to have the totalelectric charge applied listed in Table 3. Here, sample No. 45 was notsubjected to anodic electrolysis.

Subsequently, samples No. 24, 25, 28, 29, 32, 34, 36, 38, 40, 42, 44,45, and 46 were each subjected to Cr condensation treatment in thepassive film involving immersion in a 300 g/L nitric acid aqueoussolution at a temperature of 60° C. for 6 min or 15 min, to obtain astainless steel sheet for separators.

Samples No. 26 and 30 were each subjected to Cr condensation treatmentin the passive film involving electrolysis under the conditions oftemperature: 40° C., potential: 0.5 V (vs. Ag/AgCl), and electrolysistime: 1 min or 5 min using a 50 g/L nitric acid aqueous solution, toobtain a stainless steel sheet for separators. For each steel sample ID,an anode polarization curve in the electrolytic solution was measured,and the potential range in which the current density was 10 μA/cm² orless was taken to be a potential range (passivation area) in which thecorresponding stainless steel sheet was passivated. For all stainlesssteel sheets having the chemical compositions of steel sample IDs A andB (samples No. 26 and 30), a potential of 0.5 V (vs. Ag/AgCl) was in apassivation area.

Each resultant stainless steel sheet for separators was subjected toevaluation of contact resistance and corrosion resistance (evaluation ofcontact resistance and corrosion resistance before heat treatment) inthe same way as in Example 1. Moreover, each stainless steel sheet forseparators was subjected to heat treatment of holding in an airatmosphere at 200° C. for 2 hr, assuming heat treatment in a fuel cellstack production process. Each resultant stainless steel sheet forseparators was then subjected to evaluation of contact resistance andcorrosion resistance (evaluation of contact resistance and corrosionresistance after heat treatment) in the same way as in Example 1. Theresults are shown in Table 3.

The evaluation criteria of the contact resistance before heat treatmentand the contact resistance after heat treatment are as follows. Theevaluation criteria of the corrosion resistance both before and afterheat treatment are the same as the evaluation criteria of the corrosionresistance in Example 1.

Before Heat Treatment

Good: 30 mΩ·cm² or less

Poor: more than 30 mΩ·cm²

After Heat Treatment

Excellent: 20 mΩ·cm² or less

Good: more than 20 mΩ·cm² and 30 mΩ·cm² or less

Poor: more than 30 mΩ·cm².

In addition, the average equivalent circular diameter of the fineprecipitates, the number of the fine precipitates per 1 μm² at the steelsheet surface, and the components of the fine precipitates were measuredin the same way as in Example 1. The results are shown in Table 3.

Furthermore, the ratio[Cr]/[Fe] of the atomic concentration of Crexisting in chemical form other than metal to the atomic concentrationof Fe existing in chemical form other than metal in the passive film wasmeasured by the above-mentioned method. The results are shown in Table3.

TABLE 3 Sample production conditions Anodic electrolysis Annealing Totalelectric Steel Dew Annealing Annealing charge Sample sample pointtemperature time applied No. ID Atmosphere gas composition (° C.) (° C.)(sec) (C/dm²) 23 A hydrogen 75 vol % + nitrogen 25 vol % −52 950 5 20 24hydrogen 75 vol % + nitrogen 25 vol % −52 950 5 20 25 hydrogen 75 vol% + nitrogen 25 vol % −52 950 5 20 26 hydrogen 75 vol % + nitrogen 25vol % −52 950 5 20 27 B hydrogen 75 vol % + nitrogen 25 vol % −60 950 520 28 hydrogen 75 vol % + nitrogen 25 vol % −60 950 5 20 29 hydrogen 75vol % + nitrogen 25 vol % −60 950 5 20 30 hydrogen 75 vol % + nitrogen25 vol % −60 950 5 20 31 C hydrogen 90 vol % + nitrogen 10 vol % −62 97010 20 32 hydrogen 90 vol % + nitrogen 10 vol % −62 970 10 20 33 Dhydrogen 75 vol % + nitrogen 25 vol % −58 980 10 20 34 hydrogen 75 vol% + nitrogen 25 vol % −58 980 10 20 35 F hydrogen 75 vol % + nitrogen 25vol % −55 980 10 20 36 hydrogen 75 vol % + nitrogen 25 vol % −55 980 1020 37 H hydrogen 95 vol % + nitrogen 5 vol % −53 930 5 20 38 hydrogen 95vol % + nitrogen 5 vol % −53 930 5 20 39 J hydrogen 75 vol % + nitrogen25 vol % −38 980 5 20 40 hydrogen 75 vol % + nitrogen 25 vol % −38 980 520 41 K hydrogen 75 vol % + nitrogen 25 vol % −51 950 5 20 42 hydrogen75 vol % + nitrogen 25 vol % −51 950 5 20 43 L hydrogen 95 vol % +nitrogen 5 vol % −46 950 5 20 44 hydrogen 95 vol % + nitrogen 5 vol %−46 950 5 20 45 A hydrogen 75 vol % + nitrogen 25 vol % −52 950 5 Noelectrolysis 46 N hydrogen 75 vol % + nitrogen 25 vol % −48 950 5 30Sample production conditions Cr condensation treatment in passive filmImmersion time or electrolysis time Sample No. Treatment method (min)Remarks 23 — — Example 24 Immersion  6 Example 25 Immersion 15 Example26 Electrolysis  1 Example 27 — — Example 28 Immersion  6 Example 29Immersion 15 Example 30 Electrolysis  5 Example 31 — — Example 32Immersion 15 Example 33 — — Example 34 Immersion 15 Example 35 — —Example 36 Immersion 15 Example 37 — — Example 38 Immersion 15 Example39 — — Example 40 Immersion 15 Example 41 — — Example 42 Immersion 15Example 43 — — Example 44 Immersion 15 Example 45 Immersion  6Comparative Example 46 Immersion 15 Example Fine precipitates atEvaluation result steel sheet surface Before heat treatment AverageNumber of Contact resistance equivalent fine Contact Corrosionresistance circular precipitates resistance Current Sample Precipitatediameter per Passive film value density No. components (nm) 1 μm²[Cr]/[Fe] (mΩ · cm²) Determination (μA/cm²) Determination 23 Cr, Ti 8527 1.8 14.5 Good 0.17 Good 24 Cr, Ti 85 26 2.7 15.3 Good 0.16 Good 25Cr, Ti 90 25 2.8 15.4 Good 0.15 Good 26 Cr, Ti 80 26 2.1 15.0 Good 0.16Good 27 Cr, Ti 90 24 1.8 16.8 Good 0.19 Good 28 Cr, Ti 95 24 2.5 17.2Good 0.18 Good 29 Cr, Ti 90 24 2.7 17.4 Good 0.17 Good 30 Cr, Ti 90 252.3 17.0 Good 0.17 Good 31 Cr, Ti 85 22 1.8 17.3 Good 0.16 Good 32 Cr,Ti 80 20 2.8 17.9 Good 0.15 Good 33 Cr, Ti 100 28 1.9 14.4 Good 0.15Good 34 Cr, Ti 95 26 3.3 15.3 Good 0.14 Good 35 Cr, Ti 95 25 1.9 15.2Good 0.15 Good 36 Cr, Ti 90 23 3.2 16.1 Good 0.14 Good 37 Cr, Ti 60 211.8 18.4 Good 0.17 Good 38 Cr, Ti 65 22 2.7 18.9 Good 0.16 Good 39 Cr,Ti 80 21 1.6 17.5 Good 0.25 Good 40 Cr, Ti 85 23 2.2 17.9 Good 0.23 Good41 Cr, Ti 80 25 1.8 16.3 Good 0.17 Good 42 Cr, Ti 80 24 2.8 16.9 Good0.15 Good 43 Cr, Ti 65 20 1.7 18.3 Good 0.17 Good 44 Cr, Ti 60 22 2.618.7 Good 0.16 Good 45 Cr, Ti 70 1 2.3 825.7 Poor 0.16 Good 46 Cr, Ti 9025 2.9 15.3 Good 0.15 Good Evaluation result After heat treatmentContact resistance Contact resistance Corrosion resistance value Currentdensity Sample No. (mΩ · cm²) Determination (μA/cm²) DeterminationRemarks 23 22.9 Good 0.15 Good Example 24 19.5 Excellent 0.15 GoodExample 25 19.1 Excellent 0.14 Good Example 26 19.7 Excellent 0.14 GoodExample 27 23.1 Good 0.18 Good Example 28 19.6 Excellent 0.17 GoodExample 29 19.4 Excellent 0.16 Good Example 30 19.7 Excellent 0.17 GoodExample 31 23.3 Good 0.14 Good Example 32 19.8 Excellent 0.14 GoodExample 33 22.7 Good 0.13 Good Example 34 18.0 Excellent 0.13 GoodExample 35 22.9 Good 0.14 Good Example 36 18.8 Excellent 0.13 GoodExample 37 23.6 Good 0.15 Good Example 38 19.7 Excellent 0.15 GoodExample 39 23.5 Good 0.21 Good Example 40 19.7 Excellent 0.19 GoodExample 41 23.0 Good 0.16 Good Example 42 19.4 Excellent 0.14 GoodExample 43 23.6 Good 0.16 Good Example 44 19.7 Excellent 0.15 GoodExample 45 884.2 Poor 0.15 Good Comparative Example 46 19.2 Excellent0.14 Good Example

The table reveals the following points.

(a) All Examples had desired contact resistance and corrosionresistance.

(b) Particularly in Examples No. 24, 25, 26, 28, 29, 30, 32, 34, 36, 38,40, 42, 44, and 46 subjected to the Cr condensation treatment in thepassive film so that the ratio [Cr]/[Fe] of the atomic concentration ofCr existing in chemical form other than metal to the atomicconcentration of Fe existing in chemical form other than metal at thesteel sheet surface was 2.0 or more, especially excellent contactresistance was exhibited even after heat treatment.

(c) In the sample of Comparative Example No. 45, no anodic electrolysiswas performed. Consequently, the fine precipitates containing Cr werenot exposed at the steel sheet surface, and the number of the fineprecipitates at the steel sheet surface was insufficient, so thatdesired contact resistance was not obtained.

REFERENCE SIGNS LIST

-   -   1 membrane-electrode joined body    -   2, 3 gas diffusion layer    -   4, 5 separator    -   6 air passage    -   7 hydrogen passage

The invention claimed is:
 1. A stainless steel sheet for fuel cellseparators, comprising: a chemical composition containing, in mass %, C:0.003% to 0.030%, Si: 0.01% to 1.00%, Mn: 0.01% to 1.00%, P: 0.050% orless, S: 0.030% or less, Cr: 16.0% to 32.0%, Ni: 0.01% to 1.00%, Ti:0.05% to 0.45%, Al: 0.001% to 0.200%, and N: 0.030% or less, with thebalance being Fe and inevitable impurities; and fine precipitatescontaining Cr and Ti at a steel sheet surface, wherein an averageequivalent circular diameter of the fine precipitates is 20 nm or moreand 500 nm or less, and a number of the fine precipitates existing per 1μm² at the steel sheet surface is three or more.
 2. The stainless steelsheet for fuel cell separators according to claim 1, wherein thechemical composition further contains, in mass %, one or more selectedfrom Mo: 0.01% to 2.50%, Cu: 0.01% to 0.80%, Co: 0.01% to 0.50%, and W:0.01% to 3.00%.
 3. The stainless steel sheet for fuel cell separatorsaccording to claim 2, wherein the chemical composition further contains,in mass %, one or more selected from Nb: 0.01% to 0.60%, Zr: 0.01% to0.30%, V: 0.01% to 0.30%, Ca: 0.0003% to 0.0030%, Mg: 0.0005% to0.0050%, B: 0.0003% to 0.0050%, REM: 0.001% to 0.100%, Sn: 0.001% to0.500%, and Sb: 0.001% to 0.500%.
 4. The stainless steel sheet for fuelcell separators according to claim 3, wherein a ratio [Cr]/[Fe] of anatomic concentration of Cr existing in chemical form other than metal toan atomic concentration of Fe existing in chemical form other than metalat the steel sheet surface is 2.0 or more.
 5. A production method for astainless steel sheet for fuel cell separators, comprising: preparing astainless steel sheet having the chemical composition according to claim3, as a material; subjecting the stainless steel sheet to annealing, toobtain an annealed sheet; and subjecting the annealed sheet to anodicelectrolysis, wherein an atmosphere in the annealing has a dew point of−35° C. or less and a nitrogen concentration of 1 vol % or more, and atotal electric charge applied in the anodic electrolysis is 5 C/dm² to60 C/dm².
 6. The production method for a stainless steel sheet for fuelcell separators according to claim 5, further comprising after theanodic electrolysis, subjecting the annealed sheet to Cr condensationtreatment, the Cr condensation treatment being immersion in an oxidizingsolution or electrolysis in a potential range in which the stainlesssteel sheet is passivated.
 7. The stainless steel sheet for fuel cellseparators according to claim 2, wherein a ratio [Cr]/[Fe] of an atomicconcentration of Cr existing in chemical form other than metal to anatomic concentration of Fe existing in chemical form other than metal atthe steel sheet surface is 2.0 or more.
 8. A production method for astainless steel sheet for fuel cell separators, comprising: preparing astainless steel sheet having the chemical composition according to claim2, as a material; subjecting the stainless steel sheet to annealing, toobtain an annealed sheet; and subjecting the annealed sheet to anodicelectrolysis, wherein an atmosphere in the annealing has a dew point of−35° C. or less and a nitrogen concentration of 1 vol % or more, and atotal electric charge applied in the anodic electrolysis is 5 C/dm² to60 C/dm².
 9. The production method for a stainless steel sheet for fuelcell separators according to claim 8, further comprising after theanodic electrolysis, subjecting the annealed sheet to Cr condensationtreatment, the Cr condensation treatment being immersion in an oxidizingsolution or electrolysis in a potential range in which the stainlesssteel sheet is passivated C/dm².
 10. The stainless steel sheet for fuelcell separators according to claim 1, wherein the chemical compositionfurther contains, in mass %, one or more selected from Nb: 0.01% to0.60%, Zr: 0.01% to 0.30%, V: 0.01% to 0.30%, Ca: 0.0003% to 0.0030%,Mg: 0.0005% to 0.0050%, B: 0.0003% to 0.0050%, REM: 0.001% to 0.100%,Sn: 0.001% to 0.500%, and Sb: 0.001% to 0.500%.
 11. The stainless steelsheet for fuel cell separators according to claim 10, wherein a ratio[Cr]/[Fe] of an atomic concentration of Cr existing in chemical formother than metal to an atomic concentration of Fe existing in chemicalform other than metal at the steel sheet surface is 2.0 or more.
 12. Aproduction method for a stainless steel sheet for fuel cell separators,comprising: preparing a stainless steel sheet having the chemicalcomposition according to claim 10, as a material; subjecting thestainless steel sheet to annealing, to obtain an annealed sheet; andsubjecting the annealed sheet to anodic electrolysis, wherein anatmosphere in the annealing has a dew point of −35° C. or less and anitrogen concentration of 1 vol % or more, and a total electric chargeapplied in the anodic electrolysis is 5 C/dm² to 60 C/dm².
 13. Theproduction method for a stainless steel sheet for fuel cell separatorsaccording to claim 12, further comprising after the anodic electrolysis,subjecting the annealed sheet to Cr condensation treatment, the Crcondensation treatment being immersion in an oxidizing solution orelectrolysis in a potential range in which the stainless steel sheet ispassivated C/dm².
 14. The stainless steel sheet for fuel cell separatorsaccording to claim 1, wherein a ratio [Cr]/[Fe] of an atomicconcentration of Cr existing in chemical form other than metal to anatomic concentration of Fe existing in chemical form other than metal atthe steel sheet surface is 2.0 or more.
 15. A production method for astainless steel sheet for fuel cell separators, comprising: preparing astainless steel sheet having the chemical composition according to claim1, as a material; subjecting the stainless steel sheet to annealing, toobtain an annealed sheet; and subjecting the annealed sheet to anodicelectrolysis, wherein an atmosphere in the annealing has a dew point of−35° C. or less and a nitrogen concentration of 1 vol % or more, and atotal electric charge applied in the anodic electrolysis is 5 C/dm² to60 C/dm².
 16. The production method for a stainless steel sheet for fuelcell separators according to claim 15, further comprising after theanodic electrolysis, subjecting the annealed sheet to Cr condensationtreatment, the Cr condensation treatment being immersion in an oxidizingsolution or electrolysis in a potential range in which the stainlesssteel sheet is passivated.